Low temperature firing multilayer ceramic circuit boards are known that are suitable for use with low melt temperature conductive metals, such as silver, gold and copper. They have a low thermal coefficient of expansion (TCE) and they may be formulated to be compatible with both silicon and gallium arsenide devices.
These ceramic circuit boards are made from glasses that can be fired at low temperatures of less than 1000.degree. C. The circuit boards are made by admixing finely divided selected glass particles or powders, and optional inorganic fillers, with an organic vehicle including resin, solvents, dispersants and the like. The resultant slurry is cast as a thin tape, called green tape. A circuit pattern may be screen printed onto the green tape using a conductive ink formulation of a conductive metal powder, e.g., of silver, an organic vehicle and a powdered glass, generally the same glass as that used to make the green tape.
A plurality of green tapes having printed circuits thereon can be stacked together. In such case, via holes are punched into each green tape and the vias are filled with a conductive via fill ink to connect the circuits electrically. The green tapes are then aligned, laminated under heat and pressure and then fired to remove the organic materials and to vitrify the glass. Recently the green tapes have been supported on a metal support substrate to add mechanical strength to the fired green tape stack. An added advantage to this improvement is that bonding glasses used to adhere the green tape stack to the support substrate have been found to reduce shrinkage of the green tapes in the x and y dimensions. Thus almost all of the shrinkage occurs in the z dimension and thus reduces the tolerances required between the various circuits and contacts.
More recently, embedded components, such as capacitors and resistors, have been made by screen printing capacitor or resistor inks on green tapes within a green tape stack and firing all of the stacked green tapes and component layers together. This eliminates the need for mounting discrete components on the ceramic circuit boards, and reduces the space requirements for components on the green tapes.
Lead magnesium niobate (PMN) has been used to make capacitor inks. However, since PMN alone has a high firing temperature, useful capacitor inks based on PMN, must include a sintering aid to reduce the firing temperature of the mixture to below about 900.degree. C.
However, the dielectric properties of PMN-based capacitors are highly dependent on temperature. FIG. 1 illustrates the dramatic change in dielectric permittivity for PMN with temperature over the temperature range of 20-140.degree. C., measured at 10 KHz. FIG. 1 shows that the permittivity of PMN decreases steadily with an increase in temperature. As shown in FIG. 2, the loss tangent of buried capacitors made with PMN falls rapidly between 20-60.degree. C. and continues to decline thereafter as the temperature increases. However, the dielectric constant of these capacitors is generally quite high, which is very desirable. Thus a method of reducing the temperature dependence of PMN dielectric properties while retaining their high dielectric constant, would be highly desirable.